The present invention concerns a cooling sleeve for receiving a preform produced by injection molding.
Injection molding is one of the most important processes for producing moldings or molded parts. In that case the molding material which is generally originally in the form of powder or granular material is heated, plasticised and pressed under high pressure into a suitable molding tool. The molding material hardens in the molding tool and is then removed from the opened tool.
Commercially usual PET bottles are generally produced by stretch blow molding of a hollow body preform. In that procedure the hollow body preform is produced in a first step by means of an injection molding. The stretch blow molding operation which follows the injection molding process can be effected either immediately after production of the hollow body preform or at a later time.
Production of the corresponding injection molding molds necessitates a high level of complication and expenditure, wherein the injection molding mold on the one hand has to be designed for very high pressures, while on the other hand it must also have suitably heated and/or cooled passages.
Usually an injection molding tool for the production of PET preforms comprises a multiplicity of, for example 192 cavities, into which tool cores of a suitable configuration are introduced. When the tool is closed, that is to say when the core is inserted into the corresponding cavity, a space, the so-called molding space, is formed between the core on the one hand and the cavity on the other hand. The plasticised plastic material, for example PET, is then injected into that space under high pressure. As soon as the PET preform has sufficiently cooled down the mold can be opened and the preform removed. The PET preforms have an open end at which a male screwthread is formed, and a closed end. Generally the feed of the plasticised molten material is effected by way of a sprue arranged at the outside of the closed end of the preform. Due to the production procedure involved therefore a substantially disk-shaped sprue remains at the outside of the preform in the bottom region.
To reduce the cycle times, that is to say the time from one injection operation to the next, it is already usual for the preform to be removed from the mold at a very early time, at which the preform is already solid at its outside surfaces, while however the internal region thereof, the so-called core portion, is still liquid. In that condition the preform is generally transferred into a so-called receiving plate comprising a group of cooling sleeves with receiving cavities. Thus for example in the so-called horizontal tools, that is to say those injection molding tools which open by a horizontal movement of the one tool portion relative to the other, it is usual to already open the tool mold after for example eight seconds, to introduce a receiving plate with corresponding cooling sleeves into the mold, to transfer the individual preforms into the cooling sleeve, to move the receiving plate with the preforms out of the tool, to close the mold again and to begin the next injection molding process. During the next injection molding process the previous preforms remain in the cooling sleeves which are usually cooled.
The cooling sleeves have receiving cavities with an open and a closed end, the internal shape of which substantially corresponds to the external shape of the preform. To ensure that the preform is completely received in the receiving cavity they generally have a fluid passage so arranged that a fluid, generally air, can be introduced into the receiving cavity or removed from the receiving cavity by way of the fluid passage when the receiving opening is closed, that is to say for example with the preform fitted into the receiving opening. The fluid passage is generally disposed at the closed end of the receiving cavity in the region of the central longitudinal axis of the cooling sleeve. Either the air can be sucked away by way of that fluid passage in order to suck the preform fitted into the receiving opening completely into the receiving cavity and hold it there, or air can be introduced into the receiving cavity by way of the fluid passage in order to drive the preform out of the receiving cavity after cooling thereof has occurred.
As the fluid passage has a passage opening arranged on the longitudinal axis, the result of this is that, with a preform fitted into the receiving cavity, the preform does not involve direct contact with the receiving cavity in the region of the passage opening, that is to say in its bottom region, and the result of this is that that region is cooled markedly worse than the regions which have direct contact with the receiving cavity.
As however in production of the PET preforms the sprue is arranged at the outside of the preform at the bottom, the preform, due to the sprue, is somewhat thicker precisely at the location at which it is not in direct contact with the receiving cavity, than the other portions. After removal from the injection molding mold the sprue is the hottest point of the preform as it is here that hot molten material was last supplied at the end of the post-pressure step. Therefore it is just precisely the hottest portion which at the same time is somewhat thicker and would therefore have to be cooled more greatly, that involves the least cooling in the known configurations so that cooling of the bottom region, in particular in the region of the sprue, can be the factor limiting the cooling cycle. In addition, if the preform is still very soft upon transfer, that can involve deformation of the sprue if air is evacuated by way of the fluid passage.
Particularly if air is sucked away by way of the fluid passage a part of the still liquid PET can be sucked into the passage opening of the fluid passage at the outside of the preform as soon as the preform covers the passage opening so that root-shaped “noses”, so-called “carrots” are formed, which lead to visual impairment of the PET bottle produced by means of stretch blow molding.
To reduce the cooling cycle time the attempt has therefore already been made to arrange the fluid passage in decentral relationship so that it does not end in the region of the sprue. Nonetheless here too markings occur at the outside of the preform, caused by the opening of the fluid passage. Such markings however are undesirable.